1. Field of the Invention
This invention relates to a process for producing a photosensitive printing plate, especially a photosensitive printing plate capable of giving a positive print from a positive original.
2. DESCRIPTION OF THE PRIOR ART
General, a planographic printing plate consists of an oleophilic ink-adhering portion forming an image area and a hydrophilic portion forming a non-image area. Such a printing plate is prepared, for example, by a method wherein only an image portion of a hydrophilic surface is rendered oleophilic, or by a method wherein a hydrophilic layer is formed on the surface of an oleophilic substance and the hydrophilic layer is removed only at an image area to reveal the oleophilic surface.
For instance, Japanese Patent Publication No. 27,242/69 discloses a method for producing a printing plate which involves the steps of forming a hydrophilic layer containing a nuclei substance for diffusion transfer on a support having an oleophilic surface, coating a silver halide photographic emulsion on the hydrophilic layer, forming a silver image in the hydrophilic layer by diffusion transfer, and subjecting the photosensitive plate to etch bleaching utilizing the silver image, to thereby remove the corresponding part of the hydrophilic layer and reveal the oleophilic support.
U.S. Pat. No. 3,385,701 discloses a method for producing a printing plate which comprises bringing an exposed negative material into contact with a sheet comprising a support having an oleophilic surface and a hydrophilic layer formed thereon and containing a nuclei substance for diffusion transfer, to thereby form a silver image on the hydrophilic layer, and subjecting the plate to etch bleaching to thereby remove the silver image part of the hydrophilic layer and reveal the oleophilic support.
Belgian Pat. No. 717,465 discloses a method for producing a printing plate which comprises exposing a photosensitive plate consisting of a support, such as polyethylene, which has an oily receptive surface and a coating thereon of a direct positive-type gelatin silver halide emulsion having a melting point of about 82.degree.C., developing the exposed plate to form a positive silver image, and then subjecting it to etch bleaching to thereby etch away the emulsion layer at the silver image area and reveal the oily ink receptive surface. Furthermore, Belgian Pat. No. 717,466 discloses a method for producing a printing plate which comprises exposing a photosensitive plate consisting of a supprort having a non-porous hydrophilic layer not receptive oily ink and having a contact angle of not more than 65.degree. and a coating thereon of a direct positive-type gelatin silver halide emulsion layer having a melting point of not more than 82.degree.C., developing the exposed plate to form a positive silver image, subjecting it to etch bleaching to etch away the emulsion layer at the silver image area and reveal the hydrophilic layer, whereby the image area becomes a hydrophilic layer and the photosensitive layer at the non-image area becomes and oleophilic layer to form a printing plate surface.
Another conventional technique is disclosed in U.S. Defensive Publication No. 833,274, which relates to a method for producing a printing plate which involves exposing and developing a printing plate material composed of a silver halide gelatin emulsion layer and an oleophilic support, removing the image area by etch bleaching to reveal the oleophilic portion, and utilizing the gelatin layer as a hydrophilic portion.
However, when printing is performed using these known printing plates, the hydrophilic layer tends to be exfoliated to cause contamination of impressions and it is difficult to obtain printed copies of good quality. For example, in the printing plate disclosed in Japanese Patent Publication No. 27242/69, the hydrophilic image receiving layer is directly coated on the surface of the oleophilic support, and adhesion between the surface of the support and the hydrophilic layer is not sufficient. Accordingly, during printing, the hydrophilic layer at the non-image area tends to be exfoliated, and contamination of impressions occurs as a result of the adhesion of oily ink thereto.
In an ordinary planographic printing plate in which a hydrophilic layer is directly provided on the surface of an oleophilic support and that portion of the hydrophilic layer which corresponds to the image portion is removed, thus utilizing the difference between the affinity of oily ink for the thus revealed oleophilic surface and for the hydrophilic layer, it is necessary to increase this difference in ink affinity to the greatest possible extent, and to prevent the formation of smeared impressions as a result of adhesion of ink to the non-image area. This can be achieved by increasing the hydrophilicity of the hydrophilic layer as much as possible. This, however, would result in poor adhesion between the oleophilic surface and the hydrophilic layer, which will cause exfoliation of the hydrophilic layer at the non-image area to form smeared impressions printing.
It during has long been desired in the art to overcome these difficulties and to provide printing plates which do not cause smearing. This was achieved by the technique disclosed in Japanese Patent Application No. 9762/1971, which, however, has been found to suffer from the following two defects.
In the printing plate produced by the technique of our prior application (Japanese Pat. Application No. 9762/71), (1) ink adhesion is poor at the time of inking, and impressions of good quality cannot be obtained during the early stage of printing; and (2) continued printing causes the ink to adhere to the non-image area, which in turn results in smeared impressions. As shown in FIG. 7 accompanying this specification which illustrates a conventional printing plate, a non-image area 73 consists of an oleophilic support 70 and a hydrophilic layer 71 and a hardenable gelatin emulsion layer 72 superposed on the support. On the other hand, an image area 74 results from the removal of the hydrophilic layer and the gelatin emulsion layer 72 and the revealing of the surface of the support. Accordingly, the surface of the printing plate forms a concave portion at the image area 74 and a convex portion at the non-image area where the hydrophilic layer and the emulsion layer remain unremoved, thus presenting a relief surface. It has been confirmed that when such a printing surface is directly inked, ink does not adhere stably to the image portion where the area of the convex portion is large and the area of the concave portion is small, that is to say, at fine line or small dot areas.
On the other hand, when printing is continued and the number of printed copies increases, the non-image area is gradually contaminated by the ink. This is as a result of the curing of the emulsion layer which becomes the hydrophilic printing surface, its oil receptivity is increased due to the printing ink, and consequently the ink adheres thereto and causes smeared copies.